Acronyms used herein are listed below following the detailed description. Further advances in wireless communication are being developed for 3GPP New Radio (commonly referred to as 5G) and also for centralized (or cloud) radio access networks (C-RAN) which is a recent extension to the 4G/LTE system, and there are some overlaps between these two research directions. Both are to provide high spectral efficiency and energy efficiency while reducing capital and operating expenditures as compared to currently deployed radio access systems. In relevant part two main components of the traditional radio base station, the baseband and the radio head, are physically separated to dispose the higher-maintenance baseband unit (BBU) at a centralized location while the much lower-maintenance remote radio heads (RRHs) are mounted on rooftops, towers, etc. up to several kilometers away. Typically the link between them is fibre (one in each direction) to avoid the large power losses inherent with long runs of coaxial cables.
For C-RAN systems, the front haul (FH) is defined as that transmission link between the BBU and the RRH, and is shown at FIG. 1 which is a schematic overview of an example radio environment. This link 25 is referred to as front-haul regardless of the direction the data moves to distinguish it from the backhaul link that goes between the BBU and the core network. In the 5G system the BBU 20 may or may not be co-located with a serving radio access node termed a gNB whose coverage area is delineated by the dotted line. In some deployments there may be multiple RRHs associated with a single BBU 20 or multiple interconnected BBUs, and the front haul link 25 between the BBU(s) 20 and any given RRH may be wired or wireless. The illustrated UE 10 is in direct communication with the RRH 30, which in the 5G system would be operating as a transmission/reception point (TRP) of the gNB itself. Typically the RRH 30 will not have sufficient hardware to process radio-frequency (RF) signaling to baseband and vice versa. The RRH will typically contain the base station's RF circuitry plus analog-to-digital/digital-to-analog converters, up/down converters, amplifiers filters and the like. For downlink data the BBU translates the data stream coming from the core network to a form that is suitable for transmission over the air, or close to it depending on the hardware and processing capacity of the RRH. The reverse is true on the uplink where the RRH does minimal signal processing, though in some deployments the RRH may have partial though incomplete baseband processing capability. This is the form of the data sent over the FH link 25. For C-RAN the FH link 25 is termed a Common Public Radio Interface (CPRI) and it is standardized (see www.cpri.info, last visited Dec. 7, 2016) to facilitate inter-operability of BBUs and RRHs from different manufacturers. The CPRI specification uses the terms radio equipment control (REC) and radio equipment (RE) in place of BBU and RRH, respectively, but these teachings are not limited to C-RAN and 5G systems so will use the more generic terms BBU and RRH.
The C-RAN and 5G systems are to use a much larger number of antennas than currently deployed systems such as 4G/LTE. The FH link 25 will therefore require a very large bandwidth when more and more antennas are added to the system to improve performance. For example, if a traditional LTE system has 8 transmit (TX) and 8 receive (RX) antennas, increasing this to 128 antennas will increase the bandwidth required for transmission of data between the BBU 20 and the RRH 30 by a factor of 16, all else being equal. The 5G system is expected to use even more than this number of antennas making the bandwidth problem even more acute. Bandwidth reduction on the FH link 25 is a challenge for C-RAN and 5G.
One practical problem associated with reducing bandwidth on the FH link 25 is to maintain the guarantee that data transmissions between the BBU and RRH will not have any unacceptable delay; many other specifics of signal processing and message exchange depend on a prescribed maximum latency so merely accepting a delay in the data is not a simple solution. Beamforming may reduce the bandwidth requirements, where the transmission between from the RRH to the BBU is beam-space data after a number of beams are properly selected. Many beamforming techniques are known: static cell-specific; adaptive cell-specific; averaged user-specific; instantaneous user-specific; and the like. For example, for static cell-specific beamforming each cell forms a number of orthogonal beams depending on how many antennas this cell has; this is a simple technique to implement. One key challenge in any beamforming technique involves choosing the proper beams.
Another bandwidth reduction technique is data compression which reduces the number of bits in the data transmission between the BBU and RRH. With traditional data compression the is nearly always some performance degradation, most acutely for lower numbers of bits. Typically, data compression techniques reduce bit-rate by identifying and eliminating either statistical redundancy or unnecessary information bits. These are widely used for audio and video data, but traditional data compression methods cannot be directly applied for the FH link bandwidth reduction problem because the frequency domain data is white noise such that there is no statistical redundancy and all of the bits are equally important. Typical prior art data compression methods are μ-law and A-law compression that reduce dynamic range of signal, primarily using eight bits. To reduce FH bandwidth with beamforming one needs to do so on the BBU↔RRH link without an appreciable performance degradation.
As FIG. 1 illustrates, in a practical deployment of C-RAN and 5G typically there will be several or many RRHs per BBU or group of BBUs. Increasing the number of RRHs as well as the number of antennas on those RRHs rapidly increases the amount of data to be transmitted via the front haul links 25. This increased data can create a limitation to the overall system if the front haul link is not ideal, and further can increase power consumption of the radio system.